1. Field of the Invention
The present invention relates to a oil well pipe for casing, which is not destroyed by ground subsidence.
2. Description of the Related Arts
An oil well for extracting crude oil is formed by a tubing, which is an oil well pipe for drawing up crude oil from the oil reservoir to the ground, and a casing, which is an oil well pipe for protecting the tubing. Since most oil wells are bored substantially in the vertical direction, a downward pulling frictional force acts on the oil well pipe for casing (hereinafter called oil well pipe) due to ground subsidence caused by the extraction of crude oil etc.
In the conventional oil well, however, the aforementioned frictional force acting on the oil well pipe is not always high, so that the possibility that the oil well pipe is destroyed by the frictional force is not always high. Therefore, the measures for reducing the frictional force have not been taken.
Recently, the problem of oil well pipes destroyed by ground subsidence has become serious as the oil wells become deep. The oil well pipes buried in a great depth are subjected to a high underground pressure, so that the acting frictional force is high, and the destruction of oil well pipes cannot be neglected. For this reason, the measures for reducing the frictional force acting between the ground and the oil well pipe have been taken, and the development of an oil well pipe which is not destroyed by ground subsidence has been desired.